Vol. 19: 45–53, 2013 AQUATIC BIOLOGY Published online August 21 doi: 10.3354/ab00519 Aquat Biol Overwintering survival and regrowth of the invasive plant Eichhornia crassipes are enhanced by experimental warming in winter Wenhua You, Dan Yu*, Dong Xie, Lingfei Yu The National Field Station of Lake Ecosystem of Liangzi Lake, College of Life Science, Wuhan University, Wuhan, 430072, PR China ABSTRACT: The distribution of the invasive aquatic plant Eichhornia crassipes is considered to be limited by winter survival. Therefore, winter warming, as well as characteristics of overwintering organs, are expected to affect its distribution and survival. An experiment was conducted to ana- lyze the effects of winter warming and stem base treatments (size or burial) on winter survival, regrowth and thus performance of floating or rooted plants of E. crassipes using a simulated warming system in a greenhouse. Winter warming significantly increased the percentage survival in both forms of the plant and facilitated its regrowth and clonal propagation. Stem base size played an important role in determining survival, regrowth and vegetative propagation. More- over, water cover and sediment burial of stem bases facilitated overwintering. After winter sur- vival, a larger fraction of the biomass of E. crassipes was allocated to shoots. These results suggest that, with climate warming, the invasive ability of E. crassipes will be enhanced, with distribution spreading to the north into central and north China, favouring plants with large stem bases. KEY WORDS: Biomass allocation · Sediment burial · Stem base · Survival rate · Winter warming Resale or republication not permitted without written consent of the publisher INTRODUCTION effects of extreme low temperature. During long overwintering periods, plants are exposed to multi- The distribution of the invasive aquatic plant Eich- ple environmental stresses, such as drought, flood- hornia crassipes (Mart) Solms is considered to be limited ing and ice cover (Bertrand & Castonguay 2003). In to tropical or subtropical regions (Aurand 1982, Tyn- aquatic ecosystems associated with flooding, sedi- dall 1982, Madsen et al. 1993). Based on recent mentation can cause burial of aquatic plants by sand climate change models, the distribution of E. crassipes (Lowe et al. 2010, Pan et al. 2012). Previous studies will expand into higher latitudes under climate warm- have demonstrated that burial may change abiotic ing (Hellmann et al. 2008, Rahel & Olden 2008). In- and biotic conditions such as light radiation (Brown deed, climate warming in winter is more significant 1997), temperature (Baldwin & Maun 1983, Zhang & than that in summer (Karl et al. 1993). For example, Maun 1990) and activity of pathogens (van der Put- over the past 40 yr, the mean summer temperature in ten et al. 1993, D’Hertefeldt & van der Putten 1998), China did not change significantly but the mean win- thus modifying plant morpho logy and physiology, ter temperature increased up to 0.42°C every 10 yr and affecting survival and growth (Little & Maun (Ding 1997). In the context of future climate warming, 1996, Maun 1998, Yu et al. 2002). Moreover, organic few studies have addressed the effects of winter reserves in plant tissue are critical for winter stress warming on winter survival and the distribution range tolerance and rapid regrowth (Volenec et al. 1996). of this invasive plant (Owens & Madsen 1995). Storage organs, such as stolons and stem bases, can Winter survival is a complex process that does not store soluble carbohydrates and proteins that can solely depend on a plant’s ability to endure the direct be retranslocated among inter connected ramets to *Email: [email protected] © Inter-Research 2013 · www.int-res.com 46 Aquat Biol 19: 45–53, 2013 improve the survival and growth of clonal plants which carbohydrates can be stored as energy re - under natural disturbances and environmental stresses serves to withstand low temperature stress (Madsen (Madsen et al. 1993, Stuefer & Huber 1999, Dong et et al. 1993). Owens & Madsen (1995) found that ex - al. 2010). Therefore, greater stored resources con- posure to low-temperature (below 5°C) conditions for tained in larger storage organs can be retranslocated long periods could markedly increase the mortality to attached ramets and may facilitate the biomass rate of the plants, thereby affecting the overwinter- accumulation and production of new ramets (Danck- ing capacity of E. crassipes in cold winters. werts & Gordon 1989, Baur-Höch et al. 1990, Stuefer We conducted a greenhouse experiment to investi- & Huber 1999). Although sediment burial and stor- gate the effects of simulated winter warming and age organ size may contribute to winter survival and stem base treatments (size or burial) on winter sur- regrowth of plants during the overwintering period, vival, regrowth and thus performance of floating or their effects on overwintering of plants are still largely rooted forms of Eichhornia crassipes. Specifically, we unknown, especially for invasive aquatic plants. tested the following hypotheses: (1) the percentage Biomass allocation is a fundamental aspect of the survival of E. crassipes will be improved by winter competive ability of invasive aquatic plants, which warming; (2) a larger stem base (more storage mate- often allocate a large fraction of their biomass to form rial), water cover and sediment burial of the stem canopy and to support the rapid spring growth that base (temperature protection) can facilitate winter allows them to suppress other species (Madsen 1991, survival and regrowth of E. crassipes; and (3) after Sytsma & Anderson 1993). For instance, 2 invasive winter survival, biomass allocation to shoots of E. aquatic plants, water hyacinth Eichhornia crassipes crassipes will increase to support its rapid propaga- and alligator weed Alternanthera philoxeroides, can tion and spread under winter warming conditions. form dense stands above the water surface that exclude almost all other species (Aston 1977, Julien & Bourne 1988). Villamagna & Murphy (2010) stated MATERIALS AND METHODS that greater allocation to shoots may enhance water hyacinth’s ability to shade out other aquatic plants and Study site algae in the water column. However, to our knowl- edge, little information exists on the effects of winter The experiments were conducted in glasshouses at warming on the biomass allocation pattern of regrowth Liangzi Lake in the Hubei province of south-central after winter survival for invasive aquatic plants. China (30° 05’− 30° 18’ N, 114° 21’−114° 39’ E). Liangzi Eichhornia crassipes, originating from tropical South Lake is a shallow, mesotrophic lake of intermediate America, is a mat-forming aquatic plant with floating area located on the central reaches of the Yangtze and rooted forms, and is one of the world’s most River. Average winter temperatures range from 3.0 to prevalent invasive aquatic plants, causing severe 7.0°C in this region (Liu & Yu 2009). ecological and socio-economic changes in invaded habitats (Center 1994, Villamagna & Murphy 2010). Plant material It is thermophilous and is regarded as one of the fastest-growing plants in the world when conditions In early December 2008, a large number of healthy are favorable (Abbasi & Nipaney 1986, Abbasi 1998). Eichhornia crassipes were collected from Liangzi Water hyacinth was introduced into China as an orna- Lake and cultivated in a greenhouse. One week mental plant in the early 1900s and is now widely dis- later, 480 ramets were divided into 2 subsets accord- tributed in 17 provinces or cities and causes severe ing to the size of the stem base (small: n = 360 ramets, damage in more than 10 provinces. In several other diameter = 0.65 ± 0.09 cm, length = 2.0 ± 0.38 cm; provinces, E. crassipes is still being planted but it large: n= 120 ramets, diameter = 1.0 ± 0.15 cm, length cannot overwinter (Ding et al. 1995). Previous studies = 3.5 ± 0.30 cm) and used for further experiments. have demonstrated that low winter temperature is a major limiting factor in its overwintering survival and Experimental design thus determines the northern boundary of its distri- bution (Aurand 1982, Luu & Getsinger 1990). Mature Simulated warming system in the greenhouse plants consist of shoots (leaves and petioles), stem bases (short stems) and roots, producing new plants A simulated warming system was set up in a green- apically by stolons (Abbasi 1998). The stem base is house with the south door open to produce a south− the overwintering organ of the water hyacinth, in north temperature gradient (Fig. 1A) at The National You et al.: Overwintering survival of Eichhornia crassipes 47 pli tude in 2100 (1.4−5.8°C) predicted by the IPCC (IPCC 2007). Microclimate parameters including temperature (air, water and soil), light intensity and air CO2 concentration were monitored using a weather station (PC-3, Jinzhou Sunshine Technology), and none of these parameters exhibited any significant differences along the temperature gra- dient (except for temperature) during the 3-mo experimental period (Table 1). Based on our hypotheses, we set up 2 experiments using a 2-way factorial design. Both experiments started on 20 December 2008, and ended in late March 2009. Expt 1: Floating form— simulated warming and variation in stem base size For each temperature ‘position’ in the glasshouse (A, B, C and D, Fig. 1A), 8 aquaria (50 × 50 × 45 cm) were filled with lake water (54.69 mmol m−3 total nitrogen [TN], 1.41 mmol m−3 total phosphorus [TP]) at each temperature. Four aquaria were randomly selected and planted with 10 plants with small stem bases per aquarium, and the other Fig. 1. Schematic representation of the experimental design. (A) Simulated 4 aquaria were planted with 10 plants warming system in the greenhouse; (B) stem base treatments of Eichhornia cras- sipes (stem base size for the floating form; sediment burial for the rooted form) with large stem bases per aquarium (Fig. 1B). Each aquarium was treated as a treatment unit, so each treatment was Field Station of Lake Ecosystem of Liangzi Lake, replicated 4 times.